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Exceptions to Mendel

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Exceptions to Mendel

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    1. Exceptions to Mendel

    2. NJCCCS & Objectives Distinguish between various complex inheritance patterns.

    3. Today… Distinguish between various complex inheritance patterns. Apply knowledge of complex inheritance patterns to solving genetics problems.

    4. Incomplete Dominance The inheritance of flower color in snapdragons is an example of incomplete dominance. (In such cases, we will use capital letters for both alleles, here R and R’.) Hybrids (RR’) have pink flowers, whereas the homozygotes are red (RR) or white (R’R’).The inheritance of flower color in snapdragons is an example of incomplete dominance. (In such cases, we will use capital letters for both alleles, here R and R’.) Hybrids (RR’) have pink flowers, whereas the homozygotes are red (RR) or white (R’R’).

    5. Incomplete Dominance Heterozygous Genotype intermediate phenotype between two homozygous phenotypes

    6. Incomplete Dominance: F1 X F1 Because heterozygotes can be distinguished from homozygous dominants, the distribution of phenotypes in the F2 generation (1/4 red: 1/2 pink: 1/4 white) is the same as the distribution of genotypes (1/4 RR: 1/2 RR’: 1/4 R’R’). (b) Graphical representation of the differences between flower color inheritance in snapdragons (incomplete dominance) and edible peas (dominance). Because heterozygotes can be distinguished from homozygous dominants, the distribution of phenotypes in the F2 generation (1/4 red: 1/2 pink: 1/4 white) is the same as the distribution of genotypes (1/4 RR: 1/2 RR’: 1/4 R’R’). (b) Graphical representation of the differences between flower color inheritance in snapdragons (incomplete dominance) and edible peas (dominance).

    7. Solving Incomplete Dominance Punnett Square Problems Two steps: The offspring show a third phenotype (the parents each have one, and the offspring are different from the parents). The trait in the offspring is a blend (mixing) of the parental traits.

    8. Solving Incomplete Dominance Punnett Square Problems Nomenclature same letter to represent trait ’ (prime) symbol is used to designate one variation of the trait lack of the ’ (prime) symbol designates a different variation of the trait Example: White: R’ Red: R

    9. Your task… A cross between a black blahblah bird & a white blahblah bird produces offspring that are grey. The color of blahblah birds is determined by just two alleles. What are the genotypes of the parent blahblah birds in the original cross? What is/are the genotype(s) of the grey offspring? What would be the phenotypic ratio of offspring produced by two grey blahblah birds?

    10. Your task (2)… The color of fruit for plant “X” is determined by two alleles. When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit. What are the genotypes of the parent orange-fruited plants?

    11. Additional Practice Online Practice Problems http://www.k-state.edu/biology/pob/genetics/incom.htm Bikini Bottom Genetics WS

    12. Whiteboards! What possible eye colors are there? What else do you notice about eye color? Based on your understanding of Mendelian genetics, is eye color inherited in a simple dominant/recessive manner?

    13. Human Eye Color Figure: FIGURE 12.12 Title: Human eye color Caption: At least two separate genes, each with two incompletely dominant alleles, determine human eye color. A brown-eyed man and a brown-eyed woman, each heterozygous for both genes, could have children with five different eye colors, ranging from light blue (no dominant alleles) through light brown (two dominant alleles) to almost black (all four dominant alleles).Figure: FIGURE 12.12 Title: Human eye color Caption: At least two separate genes, each with two incompletely dominant alleles, determine human eye color. A brown-eyed man and a brown-eyed woman, each heterozygous for both genes, could have children with five different eye colors, ranging from light blue (no dominant alleles) through light brown (two dominant alleles) to almost black (all four dominant alleles).

    14. Human Eye Color At least two separate genes determine eye color Each gene has two incompletely dominant alleles For Example: A brown-eyed man and a brown-eyed woman, each heterozygous for both genes, could have children with five different eye colors, ranging from light blue (no dominant alleles) through light brown (two dominant alleles) to almost black (all four dominant alleles).

    15. Multiple Alleles A species may have more than two alleles for a given characteristic. Note: Every individual still carries two alleles for a characteristic. Increases possible number of genotypes and phenotypes.

    16. Multiple Allelism Example Blood Type three alleles: A, B, O produces four blood types: A (IA) B (IB) AB (IAIB) O (i) to fully understand, we also need to talk about ….

    17. Codominance Some alleles are always expressed (even in combination with other alleles). Heterozygotes display phenotype of both homozygote phenotypes Red cows are crossed with white cows -> roan cows (cows with red coats and white blotches)Red cows are crossed with white cows -> roan cows (cows with red coats and white blotches)

    18. Solving Codominance Punnett Square Problems Hint: The “recessive” & “dominant” traits appear TOGETHER in the phenotype of the hybrid organism Ex: red x white -> red & white spotted

    19. Solving Codominance Punnett Square Problems Nomenclature: letters & superscripts “F” for flower color FR = allele for red flowers FW = allele for white flowers FRFR x FWFW --> 100% FRFW

    20. Your task… Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull. roan: animal is red and white in color

    21. Your task (2)… A cross between a black cat & a tan cat produced a tabby pattern (black & tan fur together). What pattern of inheritance does this illustrate? How do you know? What percent of kitten would have tan fur it a tabby cat is crossed with a black cat?

    22. Codominance Read Human Variation and Blood Background. Blood Types Alleles A and B are codominant Type AB blood is seen where individual has the genotype IAIB Demonstration Human Variation and Blood Practice Problems.

    23. Antigen-carrying molecules Glycoproteins and glycolipids of unknown function The ABO blood group antigens are attached to oligosaccharide chains that project above the RBC surface. These chains are attached to proteins and lipids that lie in the RBC membrane. Molecular basis: The ABO gene indirectly encodes the ABO blood group antigens. The ABO locus has three main allelic forms: A, B, and O. The A and B alleles each encode a glycosyltransferase that catalyzes the final step in the synthesis of the A and B antigen, respectively. The A/B polymorphism arises from several SNPs in the ABO gene, which result in A and B transferases that differ by four amino acids. The O allele encodes an inactive glycosyltransferase that leaves the ABO antigen precursor (the H antigen) unmodified. Frequency of ABO blood group antigens A: 43% Caucasians, 27% Blacks, 28% Asians B: 9% Caucasians, 20% Blacks, 27% Asians A1: 34% Caucasians, 19% Blacks, 27% Asians Note: Does not include AB blood groups (1). Frequency of ABO phenotypes Blood group O is the most common phenotype in most populations. Caucasians: group O, 44%; A1, 33%; A2, 10%; B, 9%; A1B, 3%; A2B, 1% Blacks: group O, 49%; A1, 19%; A2, 8%; B, 20%; A1B, 3%; A2B, 1% Asians: group O, 43%; A1, 27%; A2, rare; B, 25%; A1B, 5%; A2B, rare Note: Blood group A is divided into two main phenotypes, A1 and A2 (1). http://www.ncbi.nlm.nih.gov/books/NBK2267/

    24. Human ABO Blood Group Application: Transfusions Figure: TABLE 12.1 Title: Human blood group characteristics Caption: Human blood group characteristicsFigure: TABLE 12.1 Title: Human blood group characteristics Caption: Human blood group characteristics

    25. Epistasis When one gene masks the effect of a different gene. Not dominant/recessive because it is a different gene locus. Dominance: A masking a Epistasis: ee masking AA or Aa

    26. Epistasis Example Coat Color: Laborador Retrievers Gene 1: Pigment Deposition EE or Ee: pigment deposited ee: no pigment deposited Gene 2: Production of Pigment BB: Black Bb: Black bb: chocolate

    28. Alike, But Not the Same Per pair, collect the following: 1 measuring tape 2 post-its/person pink for girls blue for boys

    29. How similar are you & your partner? Complete the inventory and compare. Number of noses: _____ 2nd toe longer than 1st toe: Yes___ No___ Sex: Male_____ Female _____ Middigital hair: Yes _____ No _____ Pierced ears: Yes _____ No _____ Height: ____ cm

    30. Data Collection & Analysis Enter your data at the appropriate place on each of the set axes. How do the patterns of inheritance differ? Sex is discontinuous Height is continuous

    31. Polygenic Inheritance Some characteristics show a range of continuous phenotypes instead of discrete, defined phenotypes Examples include human height, skin color, and body build, and grain color in wheat

    32. Polygenic Inheritance Phenotypes produced by polygenic inheritance are governed by the interaction of more than two genes at multiple loci Effects of dominant alleles are additive Example: Human skin color controlled by at least 3 genes each gene: pairs of incompletely dominant alleles

    33. Figure 12-25 Polygenic inheritance of skin color in humans (a) At least three separate genes, each with two incompletely dominant alleles, determine human skin color (the inheritance is actually much more complex than this). The backgrounds of each box indicate the depth of skin color expected from each genotype. (b) The combination of complex polygenic inheritance and environmental effects (especially exposure to sunlight) produces an almost infinite gradation of human skin colors.Figure 12-25 Polygenic inheritance of skin color in humans (a) At least three separate genes, each with two incompletely dominant alleles, determine human skin color (the inheritance is actually much more complex than this). The backgrounds of each box indicate the depth of skin color expected from each genotype. (b) The combination of complex polygenic inheritance and environmental effects (especially exposure to sunlight) produces an almost infinite gradation of human skin colors.

    34. Thinking about Human Variation Some human traits can be changed by human intervention and some cannot. Provide examples of each. Some traits are genetic and others are environmental but many reflect an interaction between the two. Name some traits that fall into this category and why you think they do.

    35. Going Further How could scientists investigate the relative contributions that heredity and the environment make to specific traits?

    36. The End

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